A wireless device has a transmit-receive switching circuit for switching between a transmission signal transmitted from an antenna and a reception signal received by the antenna. The transmit-receive switching circuit is required to be isolated from a receiver circuit in the case of transmission, and to be isolated from a transmitter circuit in the case of reception. Thus, the transmit-receive switching circuit generally includes a ¼-wavelength transfer line. When one end of this transfer line is short-circuited, impedance on the one end side becomes infinite when seen from the other end side, which makes it possible to be isolated relatively easily.
However, the impedance becomes infinite only in an ideal state. Actually, increase in on-resistance or increase in signal loss in the case of transmission is caused due to parasitic capacitance or parasitic resistance.
Since there is a trade-off relationship between parasitic capacitance and parasitic resistance, reduction in parasitic capacitance leads to increase in on-resistance, and increase in parasitic capacitance leads to increase in signal loss in the case of transmission.